The invention relates to the preparation of unsaturated polyesters from 2-methyl-1,3-propanediol (MPD). In particular, the unsaturated polyester prepared has a high fumarate content.
Unsaturated polyesters are condensation polymers with a polyester backbone formed from a glycol and an unsaturated diacid. Commonly used unsaturated diacids are maleic acid, fumaric acid, and maleic anhydride. Even though fumaric acid offers many advantages in production of unsaturated polyesters, it is seldom used because it is expensive. Saturated diacids are often used with the unsaturated diacid to control the degree of unsaturation and to modify the physical properties of the resulting polyester. For instance, the inclusion of phthalic anhydride reduces the tendency of the unsaturated polyester to crystallize and thereby improves its solubility in styrene.
Unsaturated polyesters are crosslinked, through the unsaturation, with ethylenic monomers such as styrene. To cure well with styrene, the unsaturated polyester needs a high degree of fumarate unsaturation (fumarate/maleate ratio greater than 90/10). Maleate-containing polyesters do not readily cure with styrene. However, most unsaturated polyesters are commercially made from maleic anhydride. Thus, it is crucial to effectively isomerize maleate to fumarate during the condensation polymerization.
Many glycols are used for making unsaturated polyesters. Examples are ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, and neopentyl glycol. The degree of isomerization of maleate to fumarate largely depends on the glycol used. In general, the use of a primary glycol leads to a low degree of isomerization, while the use of a secondary glycol leads to a high degree of isomerization. For instance, the esterification of maleic anhydride with neopentyl glycol, ethylene glycol, and propylene glycol gives fumarate/maleate ratios of 50/50, 75/25, and 93/7, respectively. See Ind. Eng. Chem. Prod. Res. Dev. 3(3), 218 (1964). Although propylene glycol gives a high fumarate content, it has lower reactivity toward condensation and the resultant unsaturated polyester often has a dark color and poor appearance.
2-Methyl-1,3-propanediol (MPD) became commercially available only within the last decade. It is an easily handled liquid, it has a high boiling point, and it has two primary hydroxyl groups for rapid condensation. However, like other primary glycols, MPD disadvantageously gives unsaturated polyesters having low fumarate/maleate ratios (60/40 to 70/30). Many efforts have been made to increase the fumarate content of unsaturated polyesters made from MPD. One approach is to increase the polymerization temperature. However, increasing reaction temperature often causes color problems in the product.
Co-pending application Ser. No. 09/946,326 teaches a process for making unsaturated polyesters from MPD that have fumarate/maleate ratios greater than 85/15. However, the process requires the use of propylene glycol to boost the isomerization of maleate to fumarate in a late stage of the polymerization. In sum, a better way to make a MPD-based unsaturated polyester is needed. Ideally, the unsaturated polyester would have a high fumarate content.
The invention is a process for making unsaturated polyesters from 2-methyl-1,3-propanediol (MPD). The process comprises two steps. First, one equivalent of an aromatic dicarboxylic acid derivative reacts with about two equivalents of 2-methyl-1,3-propanediol (MPD) to produce an ester diol.
Second, one equivalent of the ester diol reacts with from about 1.1 to about 1.9 equivalents of maleic anhydride. We surprisingly found that the resultant unsaturated polyester has a fumarate/maleate ratio of 90/10 or grater, which is significantly higher than the conventional unsaturated polyester prepared from MPD.
The invention also provides a novel unsaturated polyester. The unsaturated polyester consists essentially of recurring units of MPD, an aromatic dicarboxylic acid, maleic acid, and fumaric acid. It has a fumarate/maleate ratio of 90/10 or greater. The unsaturated polyester gives its thermoset polymer improved heat resistance.
The process of the invention comprises two steps. The first step involves reacting one equivalent of an aromatic dicarboxylic acid derivative with about two equivalents of 2-methyl-1,3-propanediol (MPD) to produce an ester diol. Suitable aromatic dicarboxylic acid derivatives include at least one aromatic ring and two carboxy functional groups (acids, esters, acid halides, anhydride). Examples include unsubstituted and substituted phthalic anhydrides, isophthalic acids, terephthalic acids, dialkyl terephthalates, and the like. Particularly preferred, because of their low cost and commercial availability, are phthalic anhydride, isophthalic acid, terephthalic acid, and dimethyl terephthalate. Suitable aromatic dicarboxylic acid derivatives also include recycled polyesters, especially thermoplastic polyesters such as polyethylene terephthalate (PET) or polybutylene terephthalate (PBT).
The aromatic dicarboxylic acid and MPD are preferably reacted at a temperature within the range of about 175xc2x0 C. to about 225xc2x0 C., more preferably from about 185xc2x0 C.to about 215xc2x0 C., and most preferably from about 195xc2x0 C. to about 210xc2x0 C. One advantage of the invention is that a high reaction temperature is not needed in the first step because MPD has two primary hydroxyl groups that react rapidly with the aromatic dicarboxylic acid derivatives. Lower reaction temperature gives a product with lighter color and better appearance.
Preferably, the reaction is performed under an inert atmosphere to minimize oxidative side-reactions. This is particularly important when the reaction temperature is relatively high. Preferably, a steam-jacketed reflux condenser is used. Such a condenser allows an efficient removal of water or other volatile products from the reaction mixture but keeps MPD and other reactants in the reactor. The use of a steam-jacketed reflux condenser also helps to avoid a high temperature, which otherwise is often needed to drive water out of the reaction mixture.
Optionally, an esterification or transesterification catalyst is used in the first step to accelerate the formation of the ester diol. Suitable catalysts include organotin compounds and zinc salts such as zinc acetate, zinc propionate, butyltin oxide hydroxide, dibutyltin oxide, and phenyltin oxide hydroxide. The catalyst can be used in an amount up to about 5,000 ppm based on the amount of the unsaturated polyester. Preferably, the catalyst is used in an amount from about 1 to about 500 ppm.
The equivalent ratio of MPD/aromatic dicarboxylic acid derivative is about 2/1 so that the ester diol has a low acid number. The ester diol preferably has an acid number less than about 15 mg KOH/g, more preferably less than about 10 mg KOH/g and most preferably less than about 5 mg KOH/g. The ester diol so produced has little or no color.
In the second step, one equivalent of the ester diol reacts with from about 1.1 to about 1.9 equivalents of maleic anhydride to produce an unsaturated polyester. Preferably, the equivalent ratio of maleic anhydride/ester diol is within the range of about 1.1/1 to about 1.8/1. More preferably, the ratio is from about 1.2/1 to about 1.5/1. While a sufficient amount of maleic anhydride is needed to introduce a high degree of unsaturation, using too much maleic anhydride reduces the fumarate/maleate ratio and thereby adversely reduces the reactivity of the unsaturated polyester (see Comparative Example 7).
The second step may be performed under essentially the same conditions as the first step. The reaction temperature is preferably within the range of about 175xc2x0 C. to about 225xc2x0 C., more preferably from about 185xc2x0 C. to about 215xc2x0 C., and most preferably from about 195xc2x0 C. to about 210xc2x0 C. Conventional processes for making unsaturated polyesters from MPD often need an xe2x80x9cover-cookingxe2x80x9d, i.e., heating the reaction mixture at a temperature above 220xc2x0 C., to substantially isomerize maleate to fumarate. Over-cooking often gives the product dark color and poor appearance. The process of the invention advantageously avoids over-cooking.
The catalyst from the first step may also catalyze the reaction of the second step. Alternatively, a different type or an additional amount of catalyst may be added. Suitable catalysts are discussed above. The reaction is preferably performed under an inert atmosphere to minimize oxidative side-reactions.
Optionally, the process comprises a third step, in which the unsaturated polyester from the second step is capped with a capping agent. xe2x80x9cCappingxe2x80x9d means reacting the terminal acid groups of the unsaturated polyester with a capping agent. Suitable capping agents include alcohol, glycol, olefin, monoamine, diamine, and the like, and mixtures thereof. Capping is particularly useful when the unsaturated polyester from the second step contains a high acid concentration. Reducing the acid number can reduce the viscosity of the unsaturated polyester.
Suitable alcohols for capping include C4 to C10 alcohols. Sterically bulky alcohols, such as 2-ethylhexan-1-ol, 2-methylhexan-2-ol, 3-methylpentan-3-ol, 2-methylpentan-2-ol, 3-methyl-2-butanol, 2-methylbutan-2-ol, and 3-methyl-2-butanol, are preferred. Suitable alcohols can also be ethylenically or acetylenically unsaturated, for example, 2-methyl-3-buten-2-ol and 3-methyl-1-penten-3-ol. Suitable glycols include ethylene glycol, propylene glycol, diethylene glycol, dipropylene glycol, 1,2-butanediol, 1,3-butanediol, 1,5-pentanediol, 1,6-hexanediol, MPD, 2,2-dimethyl-1,3-propanediol, cyclohexane-1,4-dimethanol, and neopentyl glycol, the like, and mixtures thereof. We have found that capping the unsaturated polyester does not change the fumarate/maleate ratio.
The invention includes an unsaturated polyester. The unsaturated polyester consists essentially of recurring units of MPD, an aromatic dicarboxylic acid, maleic acid, and fumaric acid, wherein the ratio of fumarate to maleate is 90/10 or greater. Preferably, the ratio of fumarate/maleate is about 95/5 or greater. More preferably, the ratio of fumarate/maleate is about 98/2 or greater. The higher the ratio of fumarate to maleate, the more reactive the unsaturated polyester. Usually, using MPD as a sole glycol produces unsaturated polyesters having fumarate/maleate ratios of only about 80/20 or lower. See Comparative. Example 19. These unsaturated polyesters are very difficult to cure. See application Ser. No. 09/946,326.
Preferably, the unsaturated polyester has a number average molecular weight from about 800 to about 5,000. More preferably, the molecular weight is from about 1,000 to about 4,000. Most preferably, the molecular weight is from about 1,500 to about 2,500. Preferably, the unsaturated polyester has a molecular weight distribution of less than about 5, more preferably less than about 4 and most preferably less than about 3. The narrower the molecular weight distribution, the lower the viscosity of the polyester.
The unsaturated polyester of the invention can be made by the process discussed above. In the first step, one equivalent of an aromatic dicarboxylic acid derivative reacts with about two equivalents of MPD to produce an ester diol. In the second step, one equivalent of the ester diol reacts with from about 1.1 to about 1.9, preferably from about 1.2 to about 1.5, equivalents of maleic anhydride to produce the unsaturated polyester. Optionally, the unsaturated polyester from the second step is capped with MPD. Capping with MPD increases the content of MPD recurring units in the polyester and reduces the acid number. Capping may also involve a chain extension and thereby increases the molecular weight of the polyester. However, capping does not reduce the fumarate/maleate ratio of the polyester.
Suitable aromatic dicarboxylic acids are discussed above. One example is isophthalic acid. We have surprisingly found that the isophthalic acid-based unsaturated polyester of the invention has a lower viscosity in styrene than the conventional analogue. Moreover, the thermoset polymer made from the unsaturated polyester has an improved heat-resistance, measured by DTUL (distortion temperature under load). See Table 1.
The unsaturated polyester can be free-radically cured with a vinyl monomer. Suitable vinyl monomers and free-radical initiators are described in U.S. Pat. No. 5,677,396, the teachings of which are incorporated herein by reference. Examples of vinyl monomers include unsubstituted and substituted vinyl aromatics, vinyl esters of carboxylic acids, acrylates, methacrylates, hydroxyalkyl acrylates, hydroxyalkyl methacrylates, acrylamides, methacrylamides, acrylonitrile, methacrylonitrile, alkyl vinyl ethers, allyl esters of aromatic di- and polyacids, and the like, and mixtures thereof. Preferred vinyl monomers are vinyl aromatics, halogenated vinyl aromatics, methacrylic acid esters, and diallyl esters of aromatic di- and polyacids. Particularly preferred vinyl monomers are styrene, vinyl toluene, methyl methacrylate, and diallyl phthalate.
Generally, the amount of vinyl monomer used will be within the range of about 10 to about 70 wt % based on the amount of cured thermoset. A more preferred range is from about 20 to about 65 wt %. Most preferred range is from 25 wt % to 50 wt %. The amount of vinyl monomer is altered to adjust the viscosity of the solution. A workable viscosity depends on the fabrication process. In general, the viscosity is preferably from about 100 to 2000 cps and more preferably from about 200 to about 600 cps.
Typically, a mixture of unsaturated polyester and vinyl monomer is combined with a free-radical initiator at room or elevated temperature, and is cured to give a thermoset polymer. The thermosets are often used to form composite materials. A composite usually comprises a thermoset polymer and organic or inorganic fillers including particles, pigments, and fibers (glass, carbon, nylon, and cotton).